This tab is displayed only when one of the bulk keys described here is used or when the following key is added to the AppSettings section of your web.config file:
<add key="CMSDebugViewState" value="true"/>
On the Debug -> Page ViewState tab, you can see the ViewState of particular controls on recently loaded pages. The Is dirty column indicates if the item was added to ViewState after call of the TrackViewState() method (happens typically in OnInit).
The log can be cleared using the Clear ViewState log button.
Please note: ViewState of the controls is retrieved using reflection. Therefore this debug may not work properly under medium trust environment or other specific circumstances.